The invention relates generally to a fluid friction clutch. More particularly, it relates to a fluid friction clutch having a housing with a supply chamber separated from a working chamber and with a connecting orifice in a partition between the coaxially arranged chambers, the partition capable of being opened or closed by a pivotable valve lever that is provided with a permanent magnet and stops that limit the deflection of the latter and which can be controlled via an electromagnetic coil.
German Published Application 28 04 859 discloses such a fluid friction clutch. In the disclosed clutch, an electromagnetic coil is held rotatably fixed by a ball bearing which is arranged in the cover of the clutch in front of the supply chamber. The valve lever is mounted in the supply chamber, the valve lever being provided, opposite the coil, with a permanent magnet and the valve lever is mounted on a pivot axis on the partition between the supply chamber and working chamber, the pivot axis being located close to the permanent magnet. The valve lever is loaded in the direction of the electromagnetic coil by a spring which is supported on the partition. The passage orifice in the partition is located in the outer region of the supply chamber, where, as a consequence of the rotation of the clutch housing, the clutch fluid can enter the working chamber when the valve lever is opened. In the known clutch, such as the one described above, the adjusting forces exerted by the coil, to counter the spring effect and which act on the short lever arm present between the permanent magnet and pivot axis, are relatively high. Moreover, with the arrangement of a compression spring, the adjusting forces required may change with increasing age, and, therefore, the clutch does not function satisfactorily over time.
German Published Application 27 18 723 also discloses a fluid friction clutch, in which the inflow and outflow of the working fluid are controlled by applying current to a coil. An armature arranged with a sliding fit in the partition is arranged, coaxially relative to the drive shaft of the clutch, to a valve arm. The valve arm, by means of the armature subjected to the magnetic field of the coil, can be moved axially relative to the partition between the working chamber and supply chamber, the partition being provided in its outer region with a throughflow orifice. In this clutch, the armature is held by a spring in an initial position in which the passage orifice in the partition is open. The magnetic field which is generated by the coil, and which moreover, acts via the shaft and the housing of the clutch, must therefore be relatively high. Since a return spring is also provided, the previously described disadvantages of using a spring also apply.